1. Field of the Invention
The present invention relates to a method for preparing a semiconductor substrate for use in the fabrication of a semiconductor device, and further relates to a method for fabricating a solid state imaging device.
2. Description of the Prior Art
A silicon wafer widely used as a semiconductor substrate in fabrication of such a semiconductor device as a very large scale integrated circuit (VLSI) is known as a wafer having a superhigh purity. However, the silicon wafer actually contains ppm levels of oxygen and carbon and ppt levels of heavy metal impurities such as iron and nickel or the like. Of these impurities, oxygen having the highest concentration is contained as solubilized in a supersaturated state. As a consequence, when such oxygen is subjected to a thermal history process (cooling process) immediately after the single crystalline growth by a pulling method or the like, oxygen precipitates as fine oxygen precipitate or sometimes forms secondary crystal defects (such as dislocation and stacking fault or the like), which deteriorates characteristics of the semiconductor device.
Also, the crystal immediately after having been subjected to the single crystalline growth contains, in addition to the aforementioned impurities, point defects and resultant clusters caused by the thermal fluctuations on the growth interface, which are also one of the causes of deteriorating the semiconductor device characteristics.
Of the defects present in the semiconductor crystal immediately after such a growth as mentioned above, the thermal donors (which are considered as Si.sub.x O.sub.y clusters) resulting from the oxygen precipitation are known to be removed by subjecting the crystal to an annealing process with use of an N.sub.2 gas at a temperature in a range from 700.degree. to 1000.degree. C. for a period of time in a range from 10 to 30 minutes. In general, this removal of the thermal donors is carried out after an etching process in the processing steps of the semiconductor wafer.
Meanwhile, such defects as mentioned above are present even in the fabrication of a solid state imaging device. FIG. 1 shows an example of a solid state imaging device of charge coupled device (CCD) type. The CCD type solid state imaging device is formed in the following manner. That is, within a first p-type well region 22 formed on a surface of an n-type silicon substrate 21, there are formed an n-type impurity diffusion region 23 forming a photo sensor 28, an n-type transfer channel region 24 forming a vertical register 38 and a p-type channel stop region 25. A p-type positive charge accumulation region 26 is formed on the n-type impurity diffusion region 23, while a second p-type well region 27 is formed beneath the n-type transfer channel region 24.
In this example, the photo sensor 28 is formed by a photodiode which is constituted by a PN junction between the n-type impurity diffusion region 23 and the first p-type well region 22. This photo sensor 28 is formed in correspondence with a picture element.
A transfer electrode 33 constituted by a polycrystalline silicon layer is formed on a gate insulating film 32 laying over the transfer channel region 24 and the channel stop region 25, whereby the vertical register 38 is constituted by the transfer channel region 24, the gate insulating film 32 and the transfer electrode 33.
An interlayer insulating film 34 made of phosphor-silicate glass (PSG) or the like is formed on the transfer electrode 33 and on an insulating film 29 which is formed on the positive charge accumulation region 26. A light shielding film 35 made of aluminum is selectively formed on the interlayer insulating film 34 which is formed on the transfer electrode 33. Reference numeral 37 denotes a reading gate portion.
The gate insulating film 32 of the vertical register 38 is formed of a multi-layer film which is constituted by the SiO.sub.2 film 29, an Si.sub.3 N.sub.4 film 30 and an SiO.sub.2 film 31. The insulating film 29 of the photo sensor 28 is formed by an SiO.sub.2 film.
An annealing process using nitrogen gas called the aforementioned thermal donor killer (TD killer) annealing process has effect only on the decomposition of the Si.sub.x O.sub.y clusters but has no effect on the fine oxygen precipitate, the point defect cluster or the like.
Meanwhile, a CCD type solid state imaging element shown in FIG. 2 has a silicon substrate 21 formed by a wafer made by a Czochralski (CZ) method, a magnetic-field-applied Czochralski (MCZ) method, an epitaxial wafer method or the like. In the prior art, however, fine defects caused by the oxygen introduced during crystal growth, point defects or clusters caused by the thermal fluctuations on the growth interface, or metallic impurities (especially, in the case of the epitaxial growth process) introduced from the atmosphere or ambient are already present in the silicon wafer 21 serving as a starting material. Further, secondary defects (e.g., oxygen precipitate, oxidation-included stacking fault (OSF) or dislocation or the like) are introduced during the fabricating process of the solid state imaging device. As a consequence, the aforementioned defects A, B (represented by marks x) will exist in the gate insulating film 32 of the vertical register 38 or in the photodiode portion of the photo sensor 28 in FIG. 1.
These defects, which are not less, will badly affect on the electrical characteristics of the actually fabricated solid state imaging device. That is, these defects lead to the following problems:
(1) A leak current at the interface of the PN junction is increased and a dark current is locally increased, whereby white defects (defects or error appearing as white points on the screen) as one of defects on the screen of the imaging element tend to occur (defect B).
(2) A transfer failure caused by the deterioration of breakdown voltage or deterioration of transfer characteristics or the like due to an increase in the interface state (defect A) occurs at the interface of the gate insulating film 32 and silicon substrate.